Conventionally, general electro-photographic image forming apparatus cause a photosensitive drum to be charged uniformly to initialize it, and form an electrostatic latent image on the photosensitive drum by optical writing. The electrostatic latent image is caused to be a toner image, and the toner image is directly or indirectly transferred on a transfer material like paper, and is fixed by a fixing device.
For such charging and transferring, corona discharging electrifiers and transfer devices were popularly used. However, because of the issue of deterioration of a use environment originating from generation of ozone, contact-type electrifiers and transfer devices are often used recently. For example, roller-type transfer devices are often used as the contact-type transfer devices.
In direct transferring, the roller-type transfer devices contact the rear face of a conveyance belt which contacts a peripheral face of a photosensitive drum to convey paper, and apply a transfer voltage to the conveyance belt, thereby transferring a toner image on the peripheral face of the photosensitive drum onto the paper.
Moreover, in recent days, for example, as is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-095567, indirect-transfer type image forming apparatus using an intermediate transfer belt is widely used. According to the indirect transfer scheme, first, a toner image on the peripheral face of a photosensitive drum is primary transferred on the intermediate transfer belt, and is secondary transferred on paper from the intermediate transfer belt.
According to the scheme using such intermediate transfer belt, when a strip-shaped long and thin image parallel to the driving direction of the intermediate transfer belt is primary transferred, a phenomenon in which merely the middle portion of the long and thin image is not transferred and remains at the photosensitive drum side frequently happens.
That is, a failure which is so-called “character missing” that the center portion of the long and thin image is missing and only portions by what corresponds to two parallel lines are transferred occurs. Moreover, failures which happen at the time of primary transfer include transfer dust, transfer failure, and an image defect like a whitened dot.
An image forming apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2005-10692 employs a structure of moving a toner toward an intermediate transfer body side across a contact part between an image support body and the intermediate transfer body, and the image support body comprises an electro-photographic photosensitive element having a photosensitive element layer and a surface passivation layer stacked on a conductive basal body in this order. This patent literature also discloses that the surface passivation layer of the electro-photographic photosensitive element is formed of a radiation cross-linking agent and of an electrical charge transporting material, and is subjected to radiation cross-linking, thereby improving or eliminating failures including transfer dust, transfer failure, etc.
Unexamined Japanese Patent Application KOKAI Publication No. 2004-94037 discloses an image forming apparatus which can surely suppress any image defects like a whitened dot. This image forming apparatus uses a transfer roller or a transfer belt having a specific insulating layer formed on the outermost layer, and is equipped with neutralization means for eliminating electrical charges accumulated on the transfer belt when using the transfer belt.
Furthermore, Unexamined Japanese Patent Application KOKAI Publication No. 2009-47857 discloses an image forming apparatus for the purpose of suppressing any image defects in a primary-transferred toner image from an electrostatic latent image support body on an intermediate transfer body without deteriorating a secondary transfer efficiency. This image forming apparatus has an endless belt of the intermediate transfer body employing a stacked-layer structure of at least two layers, and the belt outermost surface layer has hardness higher than that of an elastic layer which is a lower layer, and the outermost surface layer is divided into plural pieces.
According to the technologies disclosed in the foregoing Unexamined Japanese Patent Application KOKAI Publication No. 2005-10692 and Unexamined Japanese Patent Application KOKAI Publication No. 2004-94037, transfer dust, transfer failure and an image defect like a whitened dot are surely improved, so that such technologies may be able to cope with character missing if devised in some measure. However, both of those technologies have a premise that a special processing is performed on a photosensitive element, so that production thereof is difficult, resulting in advance of prices.
Moreover, a technology disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2009-47857 requires a highly-advanced and difficult technology in production of an endless belt such that the endless belt of an intermediate transfer body employs not only a stacked-layer structure but also a structure in which a surface layer has hardness higher than that of a lower layer and the outermost surface layer is divided into plural pieces. This brings about advance of prices.
It is thought that a character missing occurred in a strip-shaped long and thin image originates from pressure applied to a toner, and a technology of using a lubricate like stearic acid in order to make the motion of toner at the time of transferring smooth is proposed. The fact is, however, there is no image forming apparatus which can overcome a character missing with a simple structure so far.